This invention relates to carriers for use in developer formulations which charge electroscopic powders triboelectrically. These carriers are useful in electrophotographic processes for developing latent electrostatic images in which a colored toner carried by the carrier particle is caused to be attracted from the carrier particle to develop the latent electrostatic image.
In the electrophotographic process it is necessary to use a carrier for the toner in order to produce an electrostatic charge upon the toner particles. Various kinds of developing processes are known including cascade, powder cloud and magnetic brush processes. In each of these processes it is necessary that the carrier used have certain triboelectric properties so that it is capable of imparting to the toner particles an electrostatic charge of the proper polarity and magnitude. It has been noted that when a developer mix containing toner and carrier is allowed to stand in the developing unit of a copying machine the toner loses its electrostatic charge over a period of time. When the copying machine is turned on, the first several copies made are of relatively poor quality until the triboelectric charge on the toner is again built up.
Recently, it has been found that the carrier particles can be coated with certain types of polymeric coatings to permit variation in the triboelectric properties thereof. One such method is disclosed in U.S. Pat. No. 3,811,880 to Luther C. Browning, assigned to the same assignee as this invention.
Although polymeric coatings of this type enable a certain degree of control of the triboelectric properties of the developer mix, it has been found that in use in the environment of electrophotographic reproduction machines such carrier particles are subject to aging which limits their effectiveness. Wearing away and removal of part of the polymeric coating upon the surface of the carrier particles is another problem encountered. This may result in undesired abrasion of the photoconductive surface used for imaging and also cause bias shorting.
Another problem inherent in the use of such polymeric coatings for carrier particles is the phenomenon known as "bound toner." Through a mechanism which is not clearly understood prolonged usage of developer mixes including polymeric coated carrier particles results in toner being adhered onto the surface of the coated carrier causing a decrease in the effectiveness of the toning process and hence in the overall development of the images being reproduced.
It is also known to use carriers in which a perfluoro carboxylic acid or derivative thereof is adhered to the surface of uncoated carrier particles by chemical adsorption. Such chemically treated carrier particles will impart a positive electrostatic charge to toner particles with which they are mixed. Carriers of this type are disclosed in U.S. Pat. No. 3,922,381 to Pabitra Datta, assigned to the same assignee as this invention. However, it has been found that in high speed electrophotographic reproduction machines the carrier particles coated with such perfluoro carboxylic acids are subject to abrasion, wear and flaking and the useful life thereof is impaired.